While the art of sealing two relatively moving machine parts or elements against one another is highly developed, significant problems are still encountered when these elements are, for example, elements of a metering or dosing pump handling highly abrasive materials such as highly-filled reactive synthetic resin components.
For example, in my U.S. Pat. No. 4,211,345 issued July 8, 1980 on application Ser. No. 898,775 which was filed Apr. 21, 1978 as a continuation-in-part of Ser. No. 782,962 filed Mar. 30, 1979 (U.S. Pat. No. 4,167,263), commonly assigned with the present case, some of the problems encountered with highly-rilled fluids which must be displaced by a piston reciprocatable within a cylinder, have been described.
In general, in such systems, the packing is provided on the moving element which can be a piston and is customarily disposed between two relatively removable cylindrical surfaces, i.e. the cylindrical surfaces of the piston and the cylinder.
The packing can be a stack of sealing rings composed of flexible (i.e. compressible and even elastomeric) the rings having V-profiles or cross sections and being nested one in another. Usually the V is open in the direction of the main compartment containing the abrasive fluid so that pressure in this compartment tends to spread the V and increase the sealing action.
The piston may have an angular shoulder lying in a plane perpendicular to the axis and engaging a planar surface of the end ring of the stack turned toward this shoulder. The ring at the opposite end of the stack can confront another shoulder formed on the piston.
A seal of this type has been found to be especially effective for metering and dosing piston pumps, more generally handling machines, for reaction synthetic resin components such as polyols and isocyanates which can be highly charged with fillers, such as abrasive solids, as mentioned in U.S. Pat. No. 4,211,345. The fillers may be mineral pigments, glass fibers or any solid materials customarily utilized in polyurethanes or other reaction products to be formed by the reactive components. The operation of metering or dosing units and especially piston pumps for this purpose is fully described in U.S. Pat. No. 4,211,345 which corresponds to German Patent Document No. 27 36 177.
As pointed out in the latter patent, it is advantageous to provide means for bringing the two surfaces of the piston toward each other to compensate for wear of the packing.
While this packing system has been found to be highly effective, it has the disadvantage that, with constant operation, the friction between the packing ring and the cylinder wall increases to the point that, when the piston must be moved against an elevated pressure in one of the chambers of the cylinders, the displacement force cannot be effectively supplied in the usual way. When this force, e.g. from a fluid reservoir, must be increased to overcome both friction and a chamber pressure wear of the packing rings is sharply increased.
The reason for the increased difficulty or even impossibility of retracting the piston or effecting the return stroke thereof appears to be the penetration of gas, liquid and filler particles during the pressure or forward stroke into the region of the packing. As long as only liquid penetrates this region, the pressure relief in the compartment caused by the return stroke may be sufficient to draw the liquid in large measure out of the interstices of the packing. However, with materials of high viscosity, especially materials containing a high proportion of solid fillers, the return stroke of the piston may not suffice to allow the degree of expulsion of these materials which is essential to relieve the friction forces mentioned previously. During the next stroke the problem is multiplied and eventually the packing system must be disassembled and the packing replaced or cleaned, thereby interrupting the operation of the machine.
A similar problem arises when material is retained in the packing region and the substance which is to be handled by the metering pump is changed to a component capable of reacting with the original material. In this case, the retention of significant amounts of the original material may bring about a reaction with the new substance and hardening or setting of the reaction product in the region of the packing. In this case the packing-contaminating material cannot be removed and again disassembly and replacement may be required.
It has been found that, when all of the adjacent surfaces of the packing rings have the same angle, this difficulty is most pronounced since there is little tendency even on the return stroke for material which has penetrated into the packing assembly, to be expelled.
This results in a progressive and marked increase in the friction even on the return stroke and eventually requires maintenance operations of the type described.
A similar problem arises when the metering or dosing is utilized to handle fluids at different temperatures, e.g. fluids at temperatures different by 20.degree. to 30.degree. C. In this case, the temperature effect upon the sealing rings causes contraction of the sealing elements during operation in the cold phase and the medium penetrates into the space left free by the volumetric contraction. In the warm phase, the expansion of the sealing rings prevents this medium from being effectively expelled.